The present invention is directed to a demodulation technique for extracting information from a radio frequency signal which is capable of providing either a digital of an analog representation of a frequency-modulated (F.M.) signal. The digital or analog output may be coupled to additional data processing equipment which utilizes the frequency-modulated signal in order to extract the modulated information therefrom.
Previously it has been common practice to demodulate an incoming FM signal to an analog waveform and then to convert it to a digital waveform by an analog-to-digital conversion. This procedure is subject to a relatively high degree of error and loss of information, and requires continual attention by the data processing unit which receives the demodulated information. In addition, a considerable amount of hardware equipment is required to implement this system. One other disadvantage of this type of prior system is that "out-of-range" signals require separate detector circuits, whereas the present invention can recognize out-of-range signals simply by recognizing an out-of-range digital count.
Prior systems have also employed phase-locked loop circuits for demodulation. This type of circuit tends to generate noise in every stage and, in addition, there tends to be a loss of the locking action of the circuit when there is a step change in the incoming frequency. Moreover, sampling of the signal is done only after noise has been added in the circuit. The digital frequency-modulated demodulator of the present invention overcomes these deficiencies. The necessity of using high "Q" narrow band pass filters is also eliminated with the present invention, which represents a decided advantage over prior demodulation techniques.